1. Field of the Invention
This invention relates to a photoconductive composition, an imaging method, an imaging member and a method of elevating the level of conductivity of a photoconductive material. More specifically, the compositions of this invention exhibit a photoinduced state of elevated conductivity which persists long after exposure to light is terminated. This characteristic, hereinafter also referred to as "persistent conductivity", enables the utilization of such materials in imaging systems wherein the conductivity of the photoconductive imaging layer must persist for extended periods of time after imaging of said layer. Such materials are also useful in cyclic imaging systems, since the elevated state of conductivity which persists in these selectively illuminated areas can be readily thermally erased and the imaging layer thus restored to its previous uniformly insulative state.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on an imaging surface of an imaging member by first uniformly electrostatically charging this imaging surface and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging surface are thus rendered conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which retain the electrostatic charge and thus render visible the latent image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging surface is not to be reused. This latter practice is usually followed with respect to the binder type photoconductive films (e.g. ZnO) where the photoconductive imaging layer is also an integral part of the finished copy.
In so-called "plain paper" copying systems, the latent image can be developed on a reusable photoconductive surface or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on a reusable photoconductive surface, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying system, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulative to conductive to insulative state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulative state prior to the succeeding charging sequence will result in a increase in the dark decay rate of the photoconductor. This phenomenon, commonly referred to in the art as fatigue, has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
Many materials which persist in their conductivity after illumination can also be satisfactorily used in electrophotography by simple revision of the imaging sequence. In such a revised imaging sequence, the uncharged imaging layer is initially exposed to light and shadow image and thus rendered persistently conductive in imagewise configuration in these light struck areas. After exposure, the imaged layer is electrostatically charged in the dark whereby an electrostatic charge pattern is formed on the non-conductive areas. This charge pattern can then be developed directly or transferred to another surface for development. Development can be performed by any of the standard techniques available to the art. Subsequent to transfer of the latent image from the imaging surface, the imaging layer is uniformly illuminated to dissipate any residual charge patern and then restored to its former insulative state by heating in the dark for a brief interval. The above imaging system is more comprehensively described in U.S. Pat. No. 3,545,969, which is hereby incorporated by way of reference.
Depending upon the level of such persistent conductivity, the imaging layer can be used for short term image storage similar to standard photographic films. Inorganic phosphors, such as zinc cadmium sulfide, have reportedly been used in such an imaging mode; however, due to only short lived persistence, have not received broad commercial acceptance in electrophotography. Other disadvantages frequently encountered in the use of such materials is their relative slow exposure speed and nonerasable photoinduced conductivity making them thus unsuitable for a rapid cyclic imaging process.
A number of organic photoconductive materials having persistent photoconductivity have also been disclosed in the patent literature (U.S. Pat. No. 3,113,022); however, these materials reportedly suffer many of the same inadequacies encountered in the use of the previously discussed inorganic compositions. A relatively recent reference (U.S. Pat. No. 3,512,966) reportedly discloses thermally erasable persistently conductive organic compositions suitable for use in electrophotography. This composition is prepared from a dispersion of a polymer, such as poly-N-vinylcarbazole, a dye and an activator selected from a group consisting essentially of specific carboxylic acids; carboxylic acid anhydrides; nitrophenols; and nitroanilines.
Although the above organic composition purportedly resolves many of the deficiencies heretofore present in the materials previously discussed, it still does not possess the speed and level of persistent conductivity requisite for use in an electrophotographic device where the recorded image is to be stored for periods of up to twenty-four hours prior to development.
It is, therefore, an object of this invention to remove this as well as other related deficiencies in the prior art.
A more specific object of this invention is to provide a novel photoconductive composition.
Still another of the objects of this invention is to provide a photoconductive composition capable of retention of a recorded image for extended periods of time.
A further object of this invention is to provide a photoconductive composition having both the sensitivity and image retention capacity to be suitable for use in an electrophotographic recording device.
A still further object of this invention includes the use of this photoconductive composition in an imaging method and a method of elevating the level of conductivity in organic photoconductive compositions.